Letters of Intent received in 2021

LoI 2023-2149
Black hole winds at all scales

Topics

● Identify and discuss the most relevant open issues in the field of Quasar outflows, both from the theoretical and observational perspective

● Identify and discuss the most relevant open issues in the field of stellar mass black hole outflows, both from the theoretical and observational perspective

● Identify the key similarities and differences between QSO and GBH outflows both from a theoretical and an observational point of view. Discuss the observations that can be performed in a complementary way to cover aspects that remain unstudied in each class of systems and that will help theorists to guide their studies.

Rationale

Active Galaxies drive powerful outflows to outer regions of their host galaxies. This process is often proposed to act as a feedback agent allowing therefore an interplay between the central supermassive black hole and the host galaxy. In recent years significant progress has been made in our knowledge of AGN outflows. Observational evidence for galactic scale winds has been achieved in multiple observing bands, from radio, to sub-mm and optical, from ultraviolet to X-rays. In parallel, several theoretical models of AGN winds have been proposed and developed. Thanks to the progress made by current observing facilities, there is nowadays a realm of data that is revealing new aspects of the quasar outflow phenomenon that are not completely covered by theoretical work, and that prompts for a more complete synergy between experts on observational data and theorists.

In turn, winds from stellar mass black holes provide us with the unique opportunity to witness the appearance/disappearance and the changes of the wind at timescales of hours and days. This allows us to establish the correspondence between such changes and changes in the accreting system thus shedding light on the launching mechanism of the winds and on the effect of the winds on the accreting systems themselves.

A call for a symposium comprehensive of all these aspects is very compelling and timely given the progress that is being made in this particular moment thanks to the coming of new observing facilities like ALMA, LMT (Large Millimeter Telescope), SRT (Sardinian Radio Telescope), JWST (James Webb Space Telescope), optical telescopes equipped for Integral Field Spectroscopy, and with more consolidated results from the existing ones (IRAM, LOFAR, VLBA, VLT, HST, Chandra, XMM Newton). It is becoming very clear that a synergy among multi-wavelength observers is the path to go for making real progress in the understanding of outflows. As an example of the need of joining different expertise, recent developments and results achieved by ALMA and IRAM observations on molecular outflows in Active Galaxies show that we are able to map the outflowing gas at high resolution and in some cases resolve the spatial extent of the outflowing structure. The connection of such structures with radio jets has sometimes been established but any role of faster, more energetic winds in UV and X-rays has yet to be explored from an observational perspective. Moreover, from a theoretical point of view, it is predicted that black holes may launch an X-ray energy-conserving wind that is capable of driving large scale outflows, but again, a systematic comparison to observational results is still missing. Once more, winds from galactic black holes give us here the dynamical perspective. In these systems, winds and jets have been observed to anti-correlate in most cases. Therefore, we can study the conditions under which such outflows co-exist or not to advance our understanding of the duty cycle of each outflow class, and the resultant consequences for feedback to their environments. For this reason, we think that it is timely to discuss such issues with top-class and promising young scientists, that can exchange views, ideas and proposals.

The goal of this symposium is to bring together key experts on AGN and Galactic black holes outflows both on the theoretical and observational side of the field, and to foster fruitful interaction between the two communities. The symposium is also intended to bring together experts in multi-band observations of outflows so as to cover the most recent advance in all of them, to avoid that relevant results and open issues stay confined within a single-wavelength community.
Joining the diverse expertise of the invited participants will ensure the following open questions in the field to be addressed:

1. The growing evidence for energy-conserving outflows in powerful and luminous AGN supports the idea that high-velocity winds that are launched from the nucleus evolve systematically after shocking with the ambient medium. Multi-band observational data will be reviewed and compared to theoretical predictions of state-of-the-art shocked winds model.

2. Observations of X-ray ultra fast outflows at moderate ionization represent an unexpected challenge to current theoretical models of accretion disk winds, which nowadays focus only on how fast winds are able to produce highly ionized absorption in the Fe K band. There is an urgent need for updating/developing new models that are able to explain how these lower ionization transitions arise in an accretion disk wind. The same applies to the evidence for much more complex structure of the X-ray winds that present multiple components of velocity, ionization and column density and that is not contemplated in any present model.

3. The discovery of a fast wind with the ability to expel enough mass and energy to produce feedback in an undisturbed spiral galaxy very similar to the Milky Way, together with recent indications of feedback signatures in Seyfert type AGN revealed by Integral Field Spectroscopy techniques, calls for a revision of the current understanding of galaxy evolution.

4. The existence of fast X-ray winds in sources with radio emission and sometimes, with jetted/elongated structure observed in the radio band prompts for a deeper exploration of the connection between the radio properties and the outflow. The coincidence of these two components “re-opens” the question on who’s driving who (the radio jet drives the X-ray ionized outflow or the X-ray drives the jet?)

5. Recent observations of ionized outflows in other bands (like molecular outflows and Narrow Line Region outflows) show that the phenomenon is fairly common in AGN but the connection (if any exists) with fast winds observed in UV and X-ray bands on a much inner scale needs further investigation.

6. Observations of galactic black hole winds show that winds can remove up to several times the mass being accreted on the black hole, thus opening the possibility that such winds are responsible for halting accretion or triggering accretion state changes. However, reliable estimates of the expelled gas for a large sample and a direct connection to accretion changes in the system remain to be done.

A focussed discussion on the aforementioned items has the potential to allow a much deeper understanding of the behaviour of the outflowing gas at small and large scales. We propose to advance such understanding by preparing a dynamic symposium program with realistic goals.